Thermal printing devices of the type with which the present invention is primarily concerned operate with a supply of tape arranged to receive an image and a means for transferring image onto the tape. In one form, a tape holding case holds a supply of image receiving tape and a supply of an image transfer ribbon, the image receiving tape and the transfer ribbon being passed in overlap through a printing zone of the printing device. A printing device operating with a tape holding case of this type is described for example in EP-A-0267890 (Varitronics, Inc.). Other printing devices have been made in which letters are transferred to an image receiving tape by a dry lettering or dry film impression process. In all of these printing devices, the construction of the image receiving tape is substantially the same. That is, it comprises an upper layer for receiving an image which is secured to a releaseable backing layer by a layer of adhesive. Once an image or message has been printed on the tape, it is desired to cut off that portion of the tape to enable it to be used as a label. For this purpose, it is necessary to remove the releaseable backing layer from the upper layer to enable the upper layer to be secured to a surface by means of the adhesive layer. With existing printing devices, it is difficult to remove the releaseable backing layer from the upper layer: it is necessary first to separate the closely adhered end portions of the releaseable backing layer and the upper layer, for example using a fingernail or tweezers so that the separated end portion of the releaseable backing layer can be finger gripped to peel it off the adhesive layer. This is a relatively difficult procedure and furthermore can result in the ends of the label being damaged in the process.
There have been several attempts to solve this problem. Most such attempts have sought to rely on the provision of a so-called tab cut. In these devices, a first cut is made completely through all the layers of the tape to cut off a portion of the tape and at the same time a cut is made through only one layer of the tape. This provides a "tab" which, in theory, can be peeled away reasonably easily. While a tab cut has been implemented successfully with relatively thick, stiff upper layers there are significant difficulties in implementing so-called tab cut devices for tapes of the type used in thermal printing devices, where the upper layer is generally a thin resilient polyester material. Although there have been several proposals, no such tab cut has successfully been implemented in a thermal printing device. By way of example, reference is made to EP-A-031209 which describes one attempt to form a tab cut system, which provides a cut only through the backing layer.
Reference is also made to EP-0526213 which provides a different solution to the problem of enabling the releaseable backing sheet to be removed easily by providing a cutting system which causes the end portions of the tape to separate as a result of forming a bend in the tape before cutting.
The principle of forming a bend in the tape relies upon the difference in resilience between the image receiving tape and the backing sheet. If the backing sheet lies on the inner radius of the bend, the natural resilience of the image receiving tape will cause it to lift at the cut edge and to separate from the backing sheet against the strength of the adhesive layer. However, it can be difficult to bend the tape reliably around an appropriate radius to achieve a consistent result in separation at the cut edge of a tape.